This invention is generally concerned with electronic detection systems wherein a signal unbalance or variation, indicative of the presence of an unwanted material mass or object, generates a detection signal in a sensor for activating a warning indicator.
More specifically, the invention is concerned with optimizing detection signal discrimination by eliminating physical and environmental influences that also effect detection signals in the system.
For example, in the detection of foreign metal in a non-metallic media, the material to be tested is generally passed through an area of concentrated electromagnetic field undergoing continuous sinusoidal variation. Field variation induces eddy currents in the metal resulting in a change in the field which can be measured. Since the magnetic field decreases as the square of the distance from its source, noticeable effects on the field by metal passing through may occur primarily when this metal is in proximity to the field source. The varying magnetic field will also be affected by the metal mass, conductivity, permeability, and other physical parameters, but in simple proportion rather than exponentially. As a result, if a non-metallic material to be inspected for metal contaminants in the form of bits or pieces is carried through the varying electromagnetic field at a uniform velocity, the duration of the effect of the metal on the field is related to the field geometry or source dimensions and the metal velocity, whereas the amplitude of the effect will be more relative to physical parameters of the metal. If the material to be tested for foreign bits of metal is thus carried on a constant speed non-metallic belt conveyor, metal detection may be optimized by choosing a conveyed velocity which provides a detected metal signal duration or period differing from the period of mechanical vibration frequencies associated with the detection system, motions of other metal masses in the vicinity, electrical or electromagnetic disturbances, and other factors which cause unwanted additions to the detected signal. Optimal separation of the desired pulse from all else including random circuit noise can thus be attained by selecting only those signal pulses having a period or duration related to the belt speed and detector head geometry. This characteristic periodicity or signal duration is employed in the signal conditioning scheme that is the subject of this invention. It must be recognized, however, that the pulse-like nature of the detected signal makes customary means based on continuous sinusoidal functions, such as a narrow band pass filter where frequency = 1/period, largely ineffectual since the same frequency components appear in a variety of pulse signals and in many detectors the waveform of the detected metal pulse may change with geometry of the metal. One embodiment of the present invention employs a series of linear integrators with low pass negative feedback filters to simultaneously (a) enhance the signal pulse duration as a function of amplitude and pulse length, and (b) reduce the amplitude in proportion to duration for those pulses of greater duration than established by the metal transit time. This signal conditioning passes the minimal metal detection signal with the least modification and attenuates in proportion to deviations from that signal. Signals provided by larger metal contaminants will likewise be attenuated somewhat because of the pulse stretching action and feedback of the modified integrator but cannot be less than the minimal signal because of the original similarity in pulse lengths. The low pass negative feedback also removes slowly changing fields, drifts, etc. and provides the integrator with a zero long term reference. The linear integrator also acts to attenuate sinusoidal variations, such as produced by vibration of the detector, in proportion to their frequency by selecting a conveyor speed wherein the vibrations have a period well below the pulse period. Their attenuation relative to the signal is thus maximized. By thus sacrificing some amplitude of larger detected signal pulses, the optimum separation of desired signal and unwanted background is attained utilizing the fact that all valid detected pulses will have approximately the same period and this period will differ from the unwanted signals irrespective of amplitude. To improve the separation, the integration process may be repeated, in which case the overall characteristic of the signal conditioning means will have a sinusoidal response similar to the idealized case shown in FIG. 4 of the drawing.